1. Field of the Invention
This invention relates to improvements in a rotating drum in an automatic transmission, and more particularly to a frictionally sliding section of such a rotating drum.
2. Description of the Prior Art
Many rotating elements such as clutch drums are assembled in an automatic transmission, in which most of them have not only a single function but also multi-functions. An example of such a rotating element is disclosed in U.S. Pat. No. 4,237,749 and shown in FIG. 3 of the drawings. In FIG. 3, a clutch drum 101 is arranged rotatable around a fixed shaft 102 of a transmission casing 103 and includes an inner body 104 which is rotatably mounted on the fixed shaft 102 and encases therein a multiple disc clutch 105 having clutch plates 106 as clutch elements. Additionally, the inner body 104 is formed with a cylinder 107 in which a piston 108 is slidably movably disposed. The piston 108 functions to push the multiple disc clutch 105, serving as an engaging member for the clutch plates 106. The clutch drum 101 further includes an outer body 109 fixed to the inner body 104 at an outer peripheral portion. The outer peripheral surface 109a of the outer body 109 serves as a brake drum which incorporates with a brake band 110 disposed around the outer body 109.
As discussed above, the clutch drum 101 accomplishes a variety of functions and therefore unavoidably has a function(s) which is undesirable for the other function(s). That is to say, for example, the clutch drum outer body 109 is usually formed of a steel thick plate. This largely increases the weight of the clutch drum outer body 109, thereby raising problems from view points of weight lightening, performance improvement and improvement in efficiency of obtaining space for parts.
In the case of using a metal for the material of a frictionally sliding section, the coefficient of friction of the frictionally sliding section increases as the surface roughness increases and therefore decreases as the surface roughness decreases. Also, in the brake of the automatic transmission, the coefficient of friction of the above-mentioned clutch drum 101 is a serious factor which largely affects a shift shock, a time required for shifting and the like. For example, if the clutch drum 101 is formed of aluminum or aluminum alloy, a weight-lightening for the clutch drum will be expected. However, such a clutch drum is relatively soft and therefore the surface roughness of the frictionally sliding surface in slidable contact with the brake band unavoidably changes for a relatively short period of time as a result of wear of the frictionally sliding surface caused under repetition of tightening and loosening the brake band.
Accordingly, the clutch drum outer body 109 is inevitably formed of the steel thick plate while the clutch drum inner body 104 is formed of a pressed steel thin sheet. The clutch drum inner and outer bodies 104, 109 are then welded to each other to be combined, thereby complicating the production process of the clutch drum 101. Additionally, even in the case where the clutch drum outer body is formed of a steel thick plate despite the above-discussed production difficulty, there remains a problem in that the surface roughness of the outer peripheral surface 109a of the clutch drum outer body 109 changes with a lapse of time or a repetition of tightening the brake band as shown in FIG. 4.
A similar problem is encountered around the hub section 104a. That is to say, the hub section 104a formed of the relatively soft material (aluminum alloy) makes its wear at positions in sliding contact with seal rings 111 located at the opposite sides of an oil passage 112 through which oil is supplied to the cylinder 107. This increases a rotating friction of the clutch drum 101 thereby causing a performance degradation and an increased noise level.